<!DOCTYPE html>
<html>

<head>
    <meta charset="utf-8">
    <title></title>
</head>

<body>
    <select id="s1" onchange="onch()">
			<option value ="">请选择</option>
			<option value ="https://www.baidu.com/">百度</option>
			<option value ="https://www.4399.com/">4399</option>
			<option value ="https://www.7k7k.com/">7k7k</option>
		</select>
</body>
<script type="text/javascript">
    function onch() {
        var url = document.getElementById('s1').value;
        location.href = url;
    }
</script>

</html>